Financing in Venice was getting big, and financiers were able to get around usury, which typically involved unreasonable and often unpayable interest, by calling the interest a fee on borrowing. This fee would be upfront, and unchanging unless specified by contract
Because this was a much better deal than going to the jewish moneylenders who didnt give a fuck if you couldnt pay, and who's business practices were on par with the 1930s mafia, people in old Italy were very happy with this and the practice spread to the rest of Europe